What is ASD?

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I thought maybe it would be a good idea to start this Q&A group with the basics. Hopefully, Paula will take the ball. :-)

-- Ray (rr@primenet.com), December 08, 1997


ASD stands for Anterior Segment Dysgenesis. It is a syndrome in which the front part of the eye (anterior segment) does not develop normally (dysgenesis). ASD can be found in several species, including humans, dogs and horses.

In horses, the Rocky Mountain, Kentucky Mountain and Mountain Pleasure Saddle horse breeds have exhibited a type of ASD which is color linked to the "silver dapple" color gene. This is the gene which causes black and bay horses to appear chocolate and red chocolate. Many of the horses which have inherited the silver dapple gene (this includes chestnut and modified chestnut horses) have also inherited one or two copies of the ASD gene.

ASD is caused by a semi-dominant gene (A). In horses which inherit two copies of the gene, one from each parent, the syndrome includes several and sometimes all of the ASD abnormalities:

Anterior lenticonus internum - anterior displacement of the lens nucleus;

Dyscoria - misshapen pupil, which fails to dilate after administration of a topical mydriatic drug;

Lens subluxation - displacement of the lens;

Macrocornea/megalocornea - abnormally enlarged cornea ("popeye");

Macropalpebral fissures - abnormally widened eyelid opening;

Nuclear-cortical junctional cataract - a cataract which forms during gestation at the junction between the lens nucleus and the cortex. It does not impair vision and is not progressive;

No iridocorneal drainage angle;

Peripheral cysts of the ciliary bocy and retina;

Retinal detachment;

Retinal Dysplasia - abnormal formation of the retina;

Rhegmatogenous detachment - torn and detached retina;

Retinal reattachment - a previous detachment which has reattached;

Telecanthus/hypertelorism - eyes widely separated;

Vitreous in the anterior chamber - most frequently resulting from lens subluxation, lens luxation or breakdown of the anterior vitreous face. In this case, cataracts are progressive;

Abnormally small eyes.

In horses which inherit only one ASD gene from one parent and a normal gene from the other parent (Aa), the full syndrome is not expressed. These horses will exhibit cysts of the ciliary body, and sometimes retinal abnormalities which are associated with the cysts, but will not show other abnormalities. These horses are sometimes called "carriers" and have no visual impairment as a result of this condition.

About 20% of horses with two copies of the ASD gene (AA) have significant visual impairment from displaced lenses and cataracts. Horses with steeply-curved corneas are nearsighted. There does not appear to be pain, discomfort or inflammation associated with the condition.

-- Paula Morgan (copco@webtv.net), December 08, 1997.

In the thread entitled Discussion:Breeding Protocol, Carol Fred states that she believes Dr. Ramsey has adjusted his statistics for the incomplete penetrance factor from 12% to 20%. This is an important point to make in this thread also. It underscores the necessity of getting the funds raised to quickly get a genetic marker test developed, because it means that 20% of the horses that are clear eyed on exam are actually Aa, and 20% of those that have cysts are actually AA. It also underscores the necessity of parentage verification, because the determination of actual ASD status then could rest on knowing the parentage of the horse. For example, the determination that a particular horse that has cysts is really only Aa, could be determined with some certainty if one parent was a black, bay or buckskin, and therefore the resulting offspring, the horse in question, could only have one copy of the ASD gene inherited from the other parent.

-- Annette L. Gerhardt (gerhardt@sinosa.com), January 01, 1998.

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